Mode suppression waveguide hybrid junction



April 4, 1967 E. SALZBERG 3,312,913

MODE SUPPRESSION WAVEGUIDE HYBRID JUNCTION Filed July 31, 1964 5 Sheets-Sheet 1 INVENTOR EDWARD SALZBERG ATTORNEYS April 4, 1967 E. SALZBERG MODE SUPPRESSION WAVEGUIDE HYBRID JUNCTION Filed July 51, 1964 3 Sheets-Sheet 2 INVENTOR EDWARD SA LZBERG ATTORNEYS April 4, 1967 E. SALZBERG 3,312,913

MODE SUPPRESSION WAVEGUIDE HYBRID JUNCTION Filed July 51, 1964 5 Sheets-Sheet 5 INVEN TOR. EDWARD SALZBERG /zymlia, U/dwz/w ff Wm ATTORNEYS United States Patent 3,312,913 MODE SUPPRESSION WAVEGUIDE HYBRID JUNCTION Edward Salzberg, Wayland, Mass., assignor to Microwave Development Laboratories, Inc., Wellesley, Mass., a

corporation of Massachusetts Filed July 31, 1964, Ser. No. 386,491 4 Claims. (Cl. 333-) This invention relates in general to waveguide junctions and more particularly relates to hybrid junctions constructed to suppress undesired modes of wave propagation.

A waveguide hybrid as defined by Tyrrell in the Proceeding of the Institute of Radio Engineers, November 1947, at page 1295, is a waveguide circuit, having four waveguide terminals, two of the terminals acting as input ports and the other two terminals acting as output ports, the hybrid having the property that energy incident on one of the input terminals divides equally between the two output terminals with only a minute fraction of the energy escaping through the other input terminal. In the ideal waveguide hybrid, all the energy entering the input port would appear evenly divided at the output terminals with none of the energy appearing at the other input terminal.

This invention relates to waveguide hybrids of the type shown in US. Patents Nos. 2,739,287, 2,739,288, and 2,876,421. The waveguide hybrids shown in those patents are forms of a short slot microwave junction. A short slot hybrid junction of the sidewall type, in essence, is a pair of parallel hollow rectangular waveguides having a common narrow wall in which an aperture is provided to permit energy to couple from one waveguide to the other. The coupling aperture establishes a region common to both parallel waveguides whose width equals the spacing between the outer narrow waveguide walls. As the apertured region is broader than the width of either of the parallel waveguides, modes of wave propagation can occur in the apertured region that cannot occur in the narrower waveguides.

A highly important characteristic of the short slot hybrid junction is its ability, over an exceptionally wide band of frequencies, to divide incident power equally between the two output terminals. To obtain acceptable broad band operation, it has been necessary to insure that the third mode, designated the TE mode, is not permitted to propagate through the apertured region. To eliminate the TE mode, it is customary to reduce the width of the apertured region by indenting the outer narrow walls of the waveguide so that the combined width of the guide in the apertured region is not adequate to support the third mode. A less commonly used technique for suppressing the TE mode is the placement of posts at the sides of the hybrid junction to present an inductive load that suppresses the undesired mode. Further, the apertured region is provided with a wavelength reducing capacitive projection which usually takes the form of a dome protruding from one or both of the broad walls.

The indented side walls and the capacitive dome introduce complexities in the shape of the hybrid junction so that in constructing waveguide junctions of the short Patented Apr. 4, 1967 largely by the range of frequenceis over which it is specified to operate, and as the range of frequencies can be chosen to be any constricted band within a large spectrum, it is necessary to make a new casting for each ditferent specification. Unless a waveguide hybrid is purchased in large quantities, the method of casting hybrids results in high manufacturing costs. Where a relatively small quantity of a waveguide hybrid is purchased, manufacturing costs can be reduced by constructing the hybrid from two contiguous sections of standard hollow rectangular waveguide. The complex shape of the hybrid has, however, when standard hollow rectangular waveguide sections were used, required an inordinate amount of machining and fabrication to indent the sidewalls, and securing the capacitive dome within the structure often resulted in arcing under high power because of minute gaps between the dome and the waveguide wall. The manufacturing cost for small quantity production was therefore not appreciably reduced by using standard waveguide sections and the probability of malfunctioning was increased.

It is the principal object of this invention to provide a hybrid junction constructed from two contiguous sections of hollow rectangular Waveguide in which conductive plates perform the function of the indented sidewalls. It is a further object of this invention to provide a hybrid junction in which the function of the wavelength reducing dome is performed by a centrally positioned spline.

The invention can be more fully understood from a perusal of the following detailed exposition when considered in conjunction with the accompanying drawings in which:

FIG. 1 depicts a conventional waveguide short slot hybrid junction having indented sidewalls;

FIG. 2 is an exploded view of an embodiment of the invention;

FIG. 3 is a transverse cross-sectional view of the assembled embodiment of the invention;

FIG. 4 is a longitudinal cross-sectional view of the assembled embodiment of the invention; and

FIG. 5 is an exploded view of another embodiment of the invention.

Referring now to FIG. 1, there is illustrated a conventional short slot hybrid junction of the type customarily made by metal casting methods. The hybrid, in essence, comprises two waveguide channels 1 and 2 separated by a common narrow wall 3. A coupling aperture 4 is formed between the two waveguide channels by removing substantially all of the common narrow wall for a distance a of approximately one free space wavelength. At the center of the aperture, a dome 5 protrudes upwardly from the bottom wall into the hybrid. The dome acts as a wavelength reducing capacitive projection. The outer narrow walls 6 and 7 of the hybrid are indented at 8 and 9 to reduce the width of the apertured section, the apertured section being the region of the waveguide between the outer narrow walls that extends over the distance d of the coupling aperture. These indentations reduce the width of the apertured section to less than 3/2)\ where it is the free space wavelength of the highest operating frequency of the hybrid. As is well known to the art this effectively suppresses the TE mode.

For ease of exposition, it is assumed that the hybrid junction of FIG. 1 is intended for use in the X-band of the microwave spectrum. In the microwave art, X-band is understood to include those frequencies lying, roughly,

between 7.0 gc./s. (gigacycles per second) and 12.5 gc./s. Usually, it is unnecessary to employ microwave components that have a bandwidth covering the entire X-band. A typical hybrid junction provides a bandwidth of not more than 20% of its center frequency. In the X-band region, a hybrid junction would, for example, cover a band extending from 9.6 gc./s. to 10.6 gc./s.

In building microwave systems, it is customary to employ standard waveguide, i.e., waveguide having standardized dimensions, to connect the systems components. The entrance to waveguide channels 1 and 2 of the hybrid junction is made to be directly connected to standard waveguide. At the lower end of the X-band, relatively slight indentations in the hybrid junction are sufficient to preclude passage of the TE mode because the minimum dimension required to support this third order mode is not much less than the combined internal width of two standard X-band waveguides At the upper end of the X-band, however, the minimum dimension for suppressing the third mode is appreciably less than the combined width of two standard waveguides and large indentations, consequently are required to narrow the apertured section of the hybrid. Because of the large indentations, a mismatch occurs at the constricted section which is alleviated by (1) gradually tapering the outer narrow walls to the minimum required dimension for suppressing the third mode and (2) providing inductive posts 10, 11, 12, and 13 adjacent to the tapered walls.

In accordance wit-h the concepts of the present invention, two sections of hollow rectangular waveguide 20 and 21, preferably of standardized dimensions, are employed in which conductive plates 22 and 23 are placed to suppress the undesired third order mode of wave propagation. Identical apertures 24 and 25 are cut into one narrow wall of each waveguide section to provide a means for coupling energy from one guide to the other when the waveguide sections are put in abutting side-by-side relation as depicted in FIG. 3. The capacitive dome of FIG. 1 is replaced in the embodiment depicted in FIGS. 2, 3, and 4 by a rounded spline 26. To better secure the spline to the waveguide sections, apertures 24 and 25 are cut to leave a depending portion of the narrow side wall and the spline is provided with a slot 27 into which the depending portions of the guides fit. The two sections of waveguide have their abutting walls joined as by soldering or brazing and the slotted spline construction lends additional rigidity to the structure. The spline has sloping faces, such as face 27, so that the spline does not seat flush against the upper broad wall of the waveguides. By providing the inclined faces, it has been found that arcing does not occur when high power is transmitted through the junction. The spline is preferably joined to the depending wall portions by solder or by brazing. If the spline is made without sloping faces so that it seats flush against the broad wall, arcing is apt to occur where contact between the flush surfaces is imperfect.

Plates 22 and 23 are parallel to the waveguide narrow walls and each plate is displaced from the adjacent narrow wall by a distance m. As an aid to positioning the plates, slots, such as slots 28 and 29 in FIG. 2, are cut into the top and bottom broad waveguide walls. The plates are inserted in the slots and are held in position, preferably by brazing. The distance In is important as it is desired that no wave energy shall propagate in the space between a plate and its adjacent narrow wall. The spacing, m, must be maintained small enough so that it is, in effect, a waveguide beyond cut-off to undesired wave components.

Where plates 22 and 23 are separated from the adjacent narrow waveguide wall by an appreciable distance, the plates cause a mismatch to occur which results in reflection of wave energy. That mismatch can, where it exceeds the limit of toleration, be corrected by providing inductive posts, such as posts 30 and 31 of FIG. 2, adjacent to the end of each plate. The posts are preferably of uniform diameter and each post is spaced about one quarter of a wavelength M4 from the adjacent edge of the plate. By moving the posts toward and away from the narrow waveguide wall, an optimum position can be located for correcting the mismatch.

The apertures 24 and 25 which are cut into the narrow walls of the waveguide sections may be of various shapes. The conventional short slot hybrid junction usually employs a rectangular opening that is one free space wavelength long as the coupling aperture. Where the opening is made by machining procedures, it is preferred that the aperture cut into the narrow wall have its ends terminate in circular arcs. Because of the arcu-ate shape of the ends of the coupling aperture, a small adjustment in the length d of the aperture may become necessary to obtain equal outputs at the two output ports.

While a single spline 26 is depicted in the embodiment of FIGS. 2, 3, and 4, it is contemplated that two splines may be employed, the second spline also having a slot and being secured to the lower broad wall in the same manner as the upper spline.

FIG. 5 depicts an embodiment of the invention in which the capacitive spline is formed by forging. The coupling aperture 35 is contained within a plate 36, the aperture, preferably, having rounded ends. A spline 37 is formed integrally with plate 36 along one of the straight edges of the aperture. Preferably, the shape of the spline is as indicated in the drawing. Hollow rectangular waveguide sections 38 and 39 have portions of their narrow walls removed to permit the plate to fit into the notches thus formed. The plate is secured in position between the two sections of hollow rectangular guide by brazing. In all other respects, the embodiment of FIG. 5 is similar to the previously described embodiment of FIGS. 2, 3, and 4.

While two preferred embodiments of the invention are illustrated, it should be understood that changes can be made, and indeed are apparent, that do not depart from the essential nature of the invention. It is intended, therefore, that the scope of the invention be construed in accordance with the appended claims.

What is claimed is:

1. A short slot hybrid junction comprising:

a pair of hollow rectangular waveguides joined together to have a common narrow wall, the common narrow wall having an aperture extending longitudinally therein for about one free space wavelength to per- Init energy to couple from one waveguide to the other;

each waveguide having therein a conductive plate offset from and parallel to its outer narrow wall, the conductive plates causing a constriction in the width of the apertured region;

a capacitive spline extending longitudinally along the apertured region, the spline having a slot therein accommodating a depending portion of the common narrow wall, the spline having sloping faces providing tapered gaps between the faces and the adjacent waveguide broad wall; and

inductive posts in the waveguides adjacent to the ends of the conductive plates.

2. A short slot hybrid junction as in claim 1, wherein each hollow rectangular waveguide has in its broad walls longitudinal slots which are offset from and parallel to the outer narrow wall, and portions of the conductive plate are disposed in the slots.

3. A short slot hybrid junction as in claim 2, wherein each hollow rectangular waveguide has in its broad walls matching bores which are adjacent to the ends of the slots, and portions of the inductive posts are disposed in the bores.

4. A short slot hybrid junction comprising:

a pair of hollow rectangular waveguides joined together to have a common narrow wall, each waveguide having in its broad walls matching longitudinal slots which are offset from and parallel to the outer narrow wall,

a junction plate disposed in a gap between the waveguides and forming part of the common narrow wall, the junction plate having a coupled aperture extending longitudinally in the common wall for approxithe matching longitudinal slots, and the conductive plates causing a constriction in the Width of the apertured region to suppress undesired modes of wave propagation.

mately one free space wavelength, 5 a longitudinally extending spline integral with the junc- References Cum by the Exmmmr tion plate and forming a peripheral portion of the UNITED STATES PATENTS coupling aperture, the spline having sloping faces 2,739,287 3/1956 Riblet 333-10 X providing tapered gaps between the sloping faces and 2,739,288 3/1956 Riblet 333-10 X the adjacent waveguide broad walls, and 10 2,876,421 3/1959 Riblet 333----l0 X a conductive plate in each waveguide, the conductive plate being offset from and parallel to the waveguides outer wall and having portions disposed in HERMAN KARL SAALBACH, Primary Examiner. M. NUSSBAUM, Assistant Examiner. 

1. A SHORT SLOT HYBRID JUNCTION COMPRISING: A PAIR OF HOLLOW RECTANGULAR WAVEGUIDES JOINED TOGETHER TO HAVE A COMMON NARROW WALL, THE COMMON NARROW WALL HAVING AN APERTURE EXTENDING LONGITUDINALLY THEREIN FOR ABOUT ONE FREE SPACE WAVELENGTH TO PERMIT ENERGY TO COUPLE FROM ONE WAVEGUIDE TO THE OTHER; EACH WAVEGUIDE HAVING THEREIN A CONDUCTIVE PLATE OFFSET FROM AND PARALLEL TO ITS OUTER NARROW WALL, THE CONDUCTIVE PLATES CAUSING A CONSTRICTION IN THE WIDTH OF THE APERTURED REGION; A CAPACITIVE SPLINE EXTENDING LONGITUDINALLY ALONG THE APERTURED REGION, THE SPLINE HAVING A SLOT THEREIN ACCOMMODATING A DEPENDING PORTION OF THE COMMON NARROW WALL, THE SPLINE HAVING SLOPING FACES PROVIDING TAPERED GAPS BETWEEN THE FACES AND THE ADJACENT WAVEGUIDE BROAD WALL; AND INDUCTIVE POSTS IN THE WAVEGUIDES ADJACENT TO THE ENDS OF THE CONDUCTIVE PLATES. 